4,252 research outputs found
Gamma Ray Bursts as cosmological tools
The use of Gamma Ray Bursts as ``standard candles'' has been made possible by
the recent discovery of a very tight correlation between their rest frame
intrinsic properties. This correlation relates the GRB prompt emission peak
spectral energy E_peak to the energy E_gamma corrected for the collimation
angle theta_jet of these sources. The possibility to use GRBs to constrain the
cosmological parameters and to study the nature of Dark Energy are very
promising.Comment: 6 pages, 3 figures, Proceedings of the workshop 'Astrophysical
sources of high energy particles and radiation', Torun - Poland 20-24 June
2005, Ed. T. Bulik, B. Rudak, G. Madejsk
Spectral properties of long and short Gamma-Ray Bursts: comparison between BATSE and Fermi bursts
We compare the spectral properties of 227 Gamma Ray Bursts (GRBs) detected by
the Fermi Gamma Ray Burst Monitor (GBM) up to February 2010 with those of
bursts detected by the CGRO/BATSE instrument. Out of 227 Fermi GRBs, 166 have a
measured peak energy E_peak_obs of their \nuF(\nu) spectrum: of these 146 and
20 belong the long and short class, respectively. Fermi long bursts follow the
correlations defined by BATSE bursts between their E_peak_obs vs fluence and
peak flux: as already shown for the latter ones, these correlations and their
slopes do not originate from instrumental selection effects. Fermi/GBM bursts
extend such correlations toward lower fluence/peak energy values with respect
to BATSE ones whereas no GBM long burst with E_peak_obs exceeding a few MeV is
found, despite the possibility of detecting them. Again as for BATSE, 5%
of long and almost all short GRBs detected by Fermi/GBM are outliers of the
E_peak-isotropic equivalent energy ("Amati") correlation while no outlier
(neither long nor short) of the E_peak-isotropic equivalent luminosity
("Yonetoku") correlation is found. Fermi long bursts have similar typical
values of E_peak_obs but a harder low energy spectral index with respect to all
BATSE events, exacerbating the inconsistency with the limiting slopes of the
simplest synchrotron emission models. Although the short GRBs detected by Fermi
are still only a few, we confirm that their E_peak_obs is greater and the low
energy spectrum is harder than those of long ones. We discuss the robustness of
these results with respect to observational biases induced by the differences
between the GBM and BATSE instruments.Comment: 10 pages, 8 figures, submitted to A&
Evidence of two spectral breaks in the prompt emission of gamma ray bursts
The long-lasting tension between the observed spectra of gamma ray bursts
(GRBs) and the predicted synchrotron emission spectrum might be solved if
electrons do not completely cool. Evidence for incomplete cooling was recently
found in Swift GRBs with prompt observations down to 0.1 keV and in one bright
Fermi burst, GRB 160625B. Here we systematically search for evidence of
incomplete cooling in the spectra of the ten brightest short and long GRBs
observed by Fermi. We find that in 8/10 long GRBs there is compelling evidence
of a low energy break (below the peak energy) and good agreement with the
photon indices of the synchrotron spectrum (respectively -2/3 and -3/2 below
the break and between the break and the peak energy). Interestingly, none of
the ten short GRBs analysed shows a break but the low energy spectral slope is
consistent with -2/3. In a standard scenario, these results imply a very low
magnetic field in the emission region (B' ~ 10 G in the comoving frame), at odd
with expectations.Comment: 14 pages, 15 figures, in press, accepted for publication in A&
Tensor interactions and decays
We study the effects of charged tensor weak currents on the
strangeness-changing decays of the lepton. First, we use the available
information on the form factors to obtain BR when the system is produced
in an antisymmetric tensor configuration. Then, we propose a mechanism for the
direct production of the in decays. Using the current
upper limit on this decay we set a bound on the symmetric tensor interactions.Comment: 13 pages, Late
Acceleration of cosmic rays and gamma-ray emission from supernova remnant/molecular cloud associations
The gamma-ray observations of molecular clouds associated with supernova
remnants are considered one of the most promising ways to search for a solution
of the problem of cosmic ray origin. Here we briefly review the status of the
field, with particular emphasis on the theoretical and phenomenological aspects
of the problem.Comment: Invited talk at SUGAR201
The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects
We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the
Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured
redshift and spectral parameters, detected up to September 2007. We confirm the
existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to
previous studies, no sign of evolution with redshift of the Ep-Eiso correlation
(either its slope and normalisation) is found. The 76 bursts define a strong
Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with
redshifts evenly distributed along this correlation. We study possible
instrumental selection effects in the observer frame Ep,obs-Fluence plane. In
particular, we concentrate on the minimum peak flux necessary to trigger a
given GRB detector (trigger threshold) and the minimum fluence a burst must
have to determine the value of Ep,obs (spectral analysis threshold). We find
that the latter dominates in the Ep,obs-Fluence plane over the former. Our
analysis shows, however, that these instrumental selection effects do not
dominate for bursts detected before the launch of the Swift satellite, while
the spectral analysis threshold is the dominant truncation effect of the Swift
GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence
correlation defined by the pre--Swift sample could be affected by other, still
not understood, selection effects. Besides we caution about the conclusions on
the existence of the Ep,obs-Fluence correlation based on our Swift sample
alone.Comment: To appear in MNRA
Cosmological constraints with GRBs: homogeneous medium vs wind density profile
We present the constraints on the cosmological parameters obtained with the
-- correlation found with the most recent sample of
19 GRBs with spectroscopically measured redshift and well determined prompt
emission spectral and afterglow parameters. We compare our results obtained in
the two possible uniform jet scenarios, i.e. assuming a homogeneous density
profile (HM) or a wind density profile (WM) for the circumburst medium. Better
constraints on and are obtained with the
(tighter) -- correlation derived in the wind density
scenario. We explore the improvements to the constraints of the cosmological
parameters that could be reached with a large sample, 150 GRBs, in the
future. We study the possibility to calibrate the slope of these correlations.
Our optimization analysis suggests that GRBs with redshift
can be used to calibrate the -- with
a precision better than 1%. The same precision is expected for the same number
of bursts with . This result suggests that we do not
necessarily need a large sample of low z GRBs for calibrating the slope of
these correlations.Comment: 7 pages, 7 figures, submitted to A&
Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024
About 15% of Gamma Ray Bursts have precursors, i.e. emission episodes
preceding the main event, whose spectral and temporal properties are similar to
the main emission. We propose that precursors have their own fireball,
producing afterglow emission due to the dissipation of the kinetic energy via
external shock. In the time lapse between the precursor and the main event, we
assume that the central engine is not completely turned off, but it continues
to eject relativistic material at a smaller rate, whose emission is below the
background level. The precursor fireball generates a first afterglow by the
interaction with the external circumburst medium. Matter injected by the
central engine during the "quasi-quiescent" phase replenishes the external
medium with material in relativistic motion. The fireball corresponding to the
main prompt emission episode crashes with this moving material, producing a
second afterglow, and finally catches up and merges with the first precursor
fireball. We apply this new model to GRB 091024, an event with a precursor in
the prompt light curve and two well defined bumps in the optical afterglow,
obtaining an excellent agreement with the existing data.Comment: 11 pages, 6 figures, 3 tables. Accepted for publication in MNRAS,
Main Journa
Spectral analysis of Swift long GRBs with known redshift
We study the spectral and energetics properties of 47 long-duration gamma-ray
bursts (GRBs) with known redshift, all of them detected by the Swift satellite.
Due to the narrow energy range (15-150 keV) of the Swift-BAT detector, the
spectral fitting is reliable only for fitting models with 2 or 3 parameters. As
high uncertainty and correlation among the errors is expected, a careful
analysis of the errors is necessary. We fit both the power law (PL, 2
parameters) and cut--off power law (CPL, 3 parameters) models to the
time-integrated spectra of the 47 bursts, and present the corresponding
parameters, their uncertainties, and the correlations among the uncertainties.
The CPL model is reliable only for 29 bursts for which we estimate the nuf_nu
peak energy Epk. For these GRBs, we calculate the energy fluence and the rest-
frame isotropic-equivalent radiated energy, Eiso, as well as the propagated
uncertainties and correlations among them. We explore the distribution of our
homogeneous sample of GRBs on the rest-frame diagram E'pk vs Eiso. We confirm a
significant correlation between these two quantities (the "Amati" relation) and
we verify that, within the uncertainty limits, no outliers are present. We also
fit the spectra to a Band model with the high energy power law index frozen to
-2.3, obtaining a rather good agreement with the "Amati" relation of non-Swift
GRBs.Comment: 16 pages. To appear in MNRAS. Minor changes were introduced in this
last versio
Long Gamma-Ray Bursts as standard candles
As soon as it was realized that long GRBs lie at cosmological distances,
attempts have been made to use them as cosmological probes. Besides their use
as lighthouses, a task that presents mainly the technological challenge of a
rapid deep high resolution follow-up, researchers attempted to find the Holy
Grail: a way to create a standard candle from GRB observables. We discuss here
the attempts and the discovery of the Ghirlanda correlation, to date the best
method to standardize the GRB candle. Together with discussing the promises of
this method, we will underline the open issues, the required calibrations and
how to understand them and keep them under control. Even though GRB cosmology
is a field in its infancy, ongoing work and studies will clarify soon if and
how GRBs will be able to keep up to the promises.Comment: To appear in the proceedings of the 16th Annual October Astrophysics
Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N.
Gehrels & J. Nouse
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